In a groundbreaking ruling on gay rights, a US appeals court in Boston on Thursday struck down the heart of a federal act that defines marriage as a union solely of a man and a woman.
The decision would grant equal federal benefits to same-sex couples in Massachusetts and could set up a showdown over the rights of same-sex married couples before the US Supreme Court.
The US Court of Appeals for the First Circuit said in its 29-page ruling that a section of the 1996 Defense of Marriage Act violates the federal equal protection clause by unjustifiably denying married gays and lesbians federal benefits.
“Congress’ denial of federal benefits to same-sex couples lawfully married in Massachusetts has not been adequately supported by any permissible federal interest,’’ the court wrote.
The decision addresses only couples who are legally married within the First Circuit district - which includes much of New England - though it could establish a precedent for courts across the country. Legal analysts said it is bound for the country’s highest court, possibly by the fall.
The unanimous decision by a three-member panel of the appeals court was one of the most significant on gay rights since the 2003 landmark state Supreme Judicial Court decision that approved gay marriages in Massachusetts.
“What we’re dealing with is how the federal government treats people once they were already married by their states,’’ said Mary Bonauto, a lawyer for Gay & Lesbian Advocates & Defenders, or GLAD, the Boston-based advocacy group that had challenged the law on behalf of 17 plaintiffs.
“They were denied [benefits] simply because they married someone of the same sex,’’ said Bonauto, who also argued in the 2003 state court decision.
The Defense of Marriage Act was enacted in the aftermath of a Hawaii state court decision that hinted that same-sex marriage could be allowed under existing laws. Several states, including Hawaii, later passed laws defining marriage as a union between a man and a woman, as did Congress.
Attorney General Martha Coakley - who also challenged the Defense of Marriage Act, known as DOMA, in a federal case before the appeals court - said after the ruling: “DOMA is an unconstitutional law, for which there is no justification.
“This is a matter of fairness, equality, and respect for human beings.’’
Judge Michael Boudin, addressing the law’s conflict with state laws, wrote in Thursday’s decision of Congress’s “effort to put a thumb on the scales and influence a state’s decision as to how to shape its own marriage laws.’’
The 17 plaintiffs listed under GLAD’s lawsuit asserted that the federal law unfairly deprived them of advantages available to heterosexual couples, such as Social Security benefits and the right to file joint tax returns.
Coakley had argued that the law dictated the way the state could administer federally funded programs, forcing the state to discriminate against the type of marriages that it already approved. According to the marriage act, for instance, under some circumstances, gay couples could be denied Medicaid and the state could not bury the same-sex spouse of a war veteran in federally funded cemeteries.
In making its decision, the court determined that the marriage act needed a higher threshold of scrutiny because it discriminated based on sexual orientation.
“Invalidating a federal statute is an unwelcome responsibility for federal judges; the elected Congress speaks for the entire nation, its judgment and good faith being entitled to utmost respect,’’ the court said in an opinion written by Boudin.“But a lower federal court such as ours must follow its best understanding of governing precedent, knowing that in large matters the Supreme Court will correct misreadings.’’
Expecting an appeal in the Supreme Court, the court stayed Thursday’s decision until the justices act on the matter.
The country’s highest court ultimately will need to resolve the dispute, the First Circuit Court said. “We have done our best to discern the direction of these precedents, but only the Supreme Court can finally decide this unique case,’’ it said.
The US Department of Justice, which had abandoned efforts to defend the marriage act on equal protection grounds before the appeals court, would not comment on the decision Thursday. A spokesman for the White House told reporters that the administration maintains the belief that the act could not be defended on constitutional grounds.
Paul Clement, a lawyer representing a House panel called the Bipartisan Legal Advocacy Group, which agreed to continue to defend the marriage act after the administration refused, said in a statement that no decision on an appeal has been made.
“But,’’ Clement continued, “we have always been clear we expect this matter ultimately to be decided by the Supreme Court, and that has not changed.’’
The Bipartisan Legal Advocacy Group has 90 days to appeal. Coakley’s office and GLAD would then have 30 days to respond, meaning the high court could start to look at the case in the fall. The First Circuit issued a stay of its decision until a final disposition in the case.
The decision Thursday had a ripple effect not only among gay-rights groups, but within the state’s legal community. And it comes after President Obama’s historic declaration in support of same-sex marriage.
Six states as well as the District of Columbia have approved same-sex marriages, while other states have provisions for civil unions. A federal appeals court in California recently declared unconstitutional a state law that would have prohibited same-sex marriages. That same circuit court is slated to consider a similar challenge of the federal marriage act in the fall.
“I think it’s a huge advance for the cause of equal rights for same-sex marriage, not only in a literal sense but more in a sense of carrying the flag for the cause,’’ said Susan Stenger, of the Boston law firm Burns & Levinson LLP.
She added that the First Circuit court was conservative in interpreting Supreme Court decisions that define the levels of scrutiny needed for federal laws that could be seen as discriminating against a certain group.
“The First Circuit’s analysis is very fair and measured,’’ she said, “and frankly, if the Supreme Court is honest about its own precedents, it will be hard-pressed not to agree with the First Circuit’s decision.’’
US Representative Barney Frank, Mayor Thomas M. Menino, and Governor Deval Patrick all issued statements in support of the court’s decision.
The Massachusetts Family Institute, which had signed on as a friend of the court to defend the marriage act, said the decision was “unconscionable and another overreach of a Massachusetts court on the definition of marriage.’’
The institute said it expects the Supreme Court to overturn the ruling. But same-sex couples embraced the decision.
Jonathan Knight, who, with his husband, Marlin Nabors, was a plaintiff in the GLAD case, said it affirmed “that our relationship is full of the same kind of hopes, struggles, and dynamics as all other couples.’’Milton J. Valencia can be reached at MValencia@globe.com. Follow him on Twitter @MiltonValencia